The present invention relates to a device which detects the state of rotation of a drum, a roller or a motor used in, for example, an image forming apparatus, and in particular, to a rotation control device wherein a phase of detection signals showing the state of rotation is corrected.
As a conventional rotation detector used in the case of controlling rotation of a rotating body, there have been known, for example, a detector which detects the state of rotation of a drum and a roller of an image forming apparatus, and a detector used for controlling the rotational speed of a motor. In these rotation detectors, an optical encoder having a slit for detecting the state of rotation of a rotating body such as a drum is commonly used. This optical encoder having a slit is one wherein ON/OFF or light (or intensity of light) emitted from a light-emitting element at an edge of a slit is detected. by a light-receiving element, and the rotational speed of the drum is obtained based on a change of signals generated by the light-receiving element.
In addition to the optical encoder, an FG pattern encoder is also used commonly for detecting the rotational speed of a motor. The FG pattern encoder is one wherein a magnetism pattern having thereon multi-poles magnetized in the rotary direction is rotated together with a motor, and thereby induced electromotive force is produced on a circular and comb-shaped wire circuit (FG pattern) which is formed at a position facing the magnetism pattern so that the rotational speed of the motor may be detected.
The rotation detector like one explained above is required to be capable of detecting the state of rotation at a high precision. For this purpose, it is effective to enhance a detecting precision by increasing the number of pulses of detection signals which are generated in the rotation detector and show the state of rotation. For example, in the case of a certain optical encoder, plural optical encoders are arranged so that a phase of each detection signal may be deviated, and multiplication detection signal is generated from each detection signal for increasing the number of pulses. In the case of the FG pattern encoder, there is known the one wherein plural FG patterns whose phases are deviated are formed and multiplication detection signal is generated from the phase-deviated signal generated from the plural FG patterns for improving a detecting precision in a device disclosed, for example, in Japanese Patent Publication Open to Public Inspection No. 140088/1992 (hereinafter referred to as Japanese Patent O. P. I. Publication).
However, in the case of detection of the rotational speed of a drum by means of an optical encoder employing a slit, when an external disturbance such as a vibration generated inside or outside an image forming apparatus is inflicted to an optical encoder, for example, this external disturbance causes the slit to vibrate. Due to this vibration of the slit, its component is contained in detection signals undesirably when ON/OFF of light is detected at an edge of the slit. It is therefore impossible to detect the accurate rotational speed of a drum, which results in a problem that unevenness is caused in the rotational speed of the drum.
Further, in the case of a conventional electrophotographic apparatus among image forming apparatuses, for example, it generally is of the structure that a drive motor is controlled based on the results of detection made by an encoder provided on the drive motor for a photoreceptor drum, without being of the structure that the rotation of the drum is controlled based on the rotational speed of the photoreceptor drum detected directly. Therefore, it is difficult to drive the drum at higher rotational accuracy, which has been a problem.
In the case of an FG pattern encoder, there is a problem that an error is caused in a phase difference established in advance and a detecting precision is lowered similarly to the occasion of an optical encoder, for the causes such as, for example, a processing precision in forming FG patterns, an establishment of a threshold value in forming a waveform from signals generated at each FG pattern, and an amplitude difference of each signal.
For example, when forming a waveform from signals having a difference in amplitude generated at each FG pattern, generated signals contain high frequency noise components as shown in FIG. 33(a). When these signals are subjected to threshold value judgment with a ground level serving as a threshold value, glitch (chattering) is generated in the signals after waveform forming, and positions of the rise and fall are deviated, because the noise component is picked up even when the center value of the signals does not reach the ground level. Accordingly, it is impossible to judge surely that the signal has reached the ground level. For judging surely that the signal has reached the ground level, it is necessary to establish the threshold value by shifting it from the ground level. For example, when the threshold value is set to be higher than the ground level as shown in FIG. 33(b), it is possible to judge surely that the signal is not lower than the ground level. However, when the threshold value is set to be higher than the ground level, a difference of an amplitude of signals generated at FG patterns causes edges of the rise and fall of a square wave to be shifted, and phase deviation is caused in the square wave. FIG. 33(c) shows the signals formed after waveform forming of the signals having a difference of an amplitude. As shown in FIG. 33(d), even when a hysteresis is provided by establishing a threshold value for each of the rise and fall, phase deviation is caused in the square wave similarly to the occasion where the threshold value is set to be higher than the ground level.
Further, in the case of a conventional rotation detector outputting a sine wave as a detection signal such as an FG pattern encoder, a magnetic encoder and a resolver, when forming a waveform from detection signals, the duty of the signal after waveform forming does not reach 50% if offset adjustment is not conducted correctly. When such signals are use for composing multiplication signals, for example, a phase difference (jitter) is caused undesirably at the rise and fall of the signals, resulting in a problem that rotation detection at a high precision is difficult.